We are using the poorly infectious, endogenous, ecotropic C3H MuLV as a cloning vehicle for mouse genes related to malignant transformation. Such genes are acquired during spread of the MuLV in mouse cells and variants carrying transforming information can be selected by growth of infected cells in soft agar. This procedure had yielded virus stocks which induce rapid lymphocytic leukemia, reticulum cell sarcomas, undifferentiated sarcomas, ovarian adenocarcinomas and lung adenomas or adenocarcinomas. These new transforming viruses, which do not contain transforming genes related to Kirsten or Moloney MSV, are presently being characterized. Similar approaches to the isolation of new tumor genes are being used employing cells from other species, including man.